1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image on a sheet.
2. Description of the Related Art
In an electrophotographic image forming apparatus such as a printer, a copying machine, or a facsimile, a toner image formed by an image forming unit is transferred onto a sheet, and the sheet with the transferred toner image is discharged to a sheet discharge tray through a sheet discharge roller after the toner image is fixed on the sheet in a fixing unit. There is known a method in which a conveying roller pair for conveying the sheet guides the sheet in between the conveying roller pair including two rollers arranged to be opposed to each other, and the sheet is conveyed while being sandwiched with a frictional force generated between the two rollers. In order to prevent deterioration of performance in stacking sheets on the sheet discharge tray due to a curl of the sheet and insufficient stiffness (i.e., insufficient rigidity) of the sheet, stiffening rollers are often adopted as a sheet discharge roller pair for discharging the sheet from an inside of the apparatus onto the sheet discharge tray. When the sheet passes, the stiffening rollers stiffen the sheet through forming the sheet into a multi-corrugated shape in a width direction orthogonal to a sheet conveying direction. Thus, stacking performance is improved.
However, there are various kinds of sheets, and hence stiffness (rigidity) inherent in the sheets is different in each sheet. As a conventional configuration, there is often adopted a configuration in which a center distance between the two rollers opposed in the sheet discharge roller pair is set to a certain distance. When the center distance is set to the certain distance, the sheet with low stiffness is short of a stiffening amount, and hence the sheet hangs down because the sheet does not have desired stiffness. Consequently, the hanging-down sheet may push forward sheets which have been already discharged onto the sheet discharge tray. In order to cope with this situation, a stiffening force is sometimes strengthened through increasing a corrugating amount of the sheet. However, according to the configuration in which the stiffening force is strengthened, in a case of a sheet with high stiffness such as a thick sheet of paper, stiffening is strongly performed by the sheet discharge roller pair. When the stiffening is performed, due to high stiffness of the sheet, the sheet may strongly come into contact with the sheet discharge roller pair. In this case, of the sheet, on a portion strongly coming into contact with the sheet discharge roller pair and a portion not coming into contact therewith, roller marks, i.e., image density nonuniformity, sometimes occur. The roller marks are, for example, coagulation nonuniformity occurring when the roller comes into contact with the toner image which is not fully cooled, and streaks occurring when a corner of the roller comes into contact with the sheet.
There are the following problems. When the sheet has low stiffness, the stiffening amount is insufficient. When the sheet has high stiffness, the sheet tends to strongly come into contact with the sheet discharge roller pair, and thus the roller marks are likely to occur. Therefore, it has been a challenge to achieve a balance between a configuration in which stiffening is strongly performed when a sheet with low stiffness such as a thin sheet of paper is fed, and a configuration in which occurrence of the roller marks is reduced through weakly performing stiffening when a sheet with high stiffness such as a thick sheet of paper is fed.
There is proposed a technology of changing a stiffening amount of a sheet according to a kind of the sheet (see Japanese Patent Application Laid-Open No. H03-88672). In the technology described in Japanese Patent Application Laid-Open No. H03-88672, there is provided a plurality of stiffening rollers movable in an axial direction of a sheet discharge roller, and positions of the stiffening rollers are shifted by a motor according to the kind of the sheet to be discharged. Thus, the technology of imparting desired stiffness to the sheet according to the kind of the sheet is proposed.
Further, there is proposed a technology of adjusting stiffening of a sheet with a configuration in which a roller shaft body is movable (see Japanese Patent Application Laid-Open No. 2001-302060). In the technology described in Japanese Patent Application Laid-Open No. 2001-302060, in an upper sheet discharge roller and a lower sheet discharge roller constituting a sheet discharge roller pair, a plurality of star wheels is provided to a side of the upper sheet discharge roller, and rollers with rib are provided to a side of the lower sheet discharge roller. There is obtained positional relation in which outer peripheral portions of the star wheels and outer peripheral portions of the rollers with rib overlap each other when viewed in an axial direction. In the technology, a rotating member with a cam surface (tapered surface) is provided. When a rotating shaft of the lower sheet discharge roller is pressed against the cam surface and the rotating member is rotated by a motor, the lower sheet discharge roller moves in the axial direction in conjunction with the cam surface. Thus, an axial distance between the star wheels of the upper sheet discharge roller and the rollers with rib of the lower sheet discharge roller is changed, a corrugating amount of the sheet is increased or decreased, and desired stiffening is performed on the sheet according to a kind of the sheet.
In the technologies disclosed in the above-mentioned patent publications, the stiffening amount of the sheet is changed according to a material and size of the sheet with use of drive from the motor. However, according to each of the configurations, it costs to provide a mechanism for transmitting rotation of the motor, and a control circuit for controlling the motor. Further, the configuration is complicated, and hence there arises a problem in that large space is required and a size of the apparatus is increased.